Magnetic flux leakage (MFL) is a magnetic method of testing that is used to detect corrosion, erosion and pitting in steel structures, such as pipelines and storage tanks. A u-shaped magnetic circuit is used to magnetize the steel. The magnetic field “leaks” from the steel at areas where there is corrosion or missing metal. In an MFL device, a magnetic field sensor is placed between the poles of the magnetic circuit to detect the leakage field.
MFL inspection devices have been used for many years with only coils, or coil pairs, as sensors. Wire coils sense changes in magnetic field (AC component). Coils are useful, in that, the interface uses simple low-power electronics, they can be very sensitive, they are somewhat temperature stable, they sense fields inside the coil so a single coil can cover a large area, mechanically rugged, and multiple sensors can be manufactured to be very similar. However, coils only respond to changes in magnetic fields, and the size of the output signal is related to the size of the magnetic field, number of coil turns and the rate of change of the magnetic field (Faradays Law).
More recently, MFL inspection devices have used magnetoresistors (e.g., Hall effect devices), rather than coils. Magnetoresistive sensors sense absolute magnetic field levels (DC component). Magnetoresistors respond to steady state and changing magnetic fields. However, magnetoresistors interface needs are somewhat complex electronics, multiple sensors are all different and need individual calibration, sensor output changes with temperature and mechanical stress, are not sensitive to tiny magnetic fields, and require power to operate.
The inspection tools are used to find metal loss flaws for things like: railroad rails, pipelines, spherical liquid natural gas (LNG) tanks and above-ground storage tank floors and walls and the like. For example, typically, the floor of a storage tank is made by welding rectangular steel plates together. The floor is sometimes coated with fiberglass or a tar-like substance. The MFL inspection device rides over a bumpy surface on the tank floor. Most of the time, the bumps cause false signals to be seen by the magnetic field sensors. When there are many bumps, it is hard to see the signals from metal loss flaws. The operator of the MFL inspection device has to move the device back and forth over a short distance that doesn't have any bumps to see if the signal is from the flaw.
To this end, although MFL devices exist, there is a need for an improved MFL detection apparatus with improved accuracy, repeatibility and one that filters out false signals. It is to such an MFL detection apparatus and methods of making and using such apparatus and components of the apparatus that the present disclosure is directed.